Alicyclic structure-containing polymers such as norbornene polymers are polymers excellent in transparency, heat resistance, low water absorption property, chemical resistance, etc. and are hence used in a wide variety of fields as optical materials for lenses, optical disk substrates and the like; materials for medical equipments such as syringes, drug containers, bags for infusion and bags for transfusion; and food packaging materials such as food packaging bottles and food packaging films.
Although a molding formed from an alicyclic structure-containing polymer exhibits excellent various properties, it has involved a problem that when a pattern such as characters, graduation marks, figures and/or the like is printed with a printing ink on the surface thereof, the resultant ink layer is easy to separate. The ink layer is easy to undergo interfacial separation from the surface of the molding under severe environment such as high temperature and high humidity in particular. Uses of moldings formed from the alicyclic structure-containing polymer have hence been limited.
For example, uses relevant to optical materials for the alicyclic structure-containing polymers include a field of liquid crystal display devices (LCD). It has heretofore been proposed to use them as phase plates, surface protecting films, etc (Japanese Patent Application Laid-Open Nos. 4-361230 and 5-2108). As the phase plates and the surface protecting films, stretched films or unstretched films of the alicyclic structure-containing polymer may be used as they are. On the other hand, in order to use the alicyclic structure-containing polymer as a light guide for back light in LCD, it is necessary to provide a reflecting layer on the back surface of a transparent light guide formed from the alicyclic structure-containing polymer.
As the back light in LCD, is known, for example, an edge type back light. FIG. 1 is a sectional view illustrating an exemplary edge type back light. A reflecting layer 2 for irregularly reflecting light is provided on the back surface of a transparent light guide 1, and light from a light source 4 (for example, a cold cathode tube) arranged on a side surface of the light guide 1 is irregularly reflected or scattered by the reflecting layer and gathered on the side of a liquid crystal display panel (provided at an upper position in the drawing). A diffusing plate 3 is arranged on the surface of the light guide 1 on the side of the liquid crystal display panel, as needed, and a converging plate (not illustrated) may be further arranged thereon in some cases. A reflecting plate may be arranged below the reflecting layer as needed. Such an edge type back light may be made thin and/or large size.
As a means for providing a reflecting layer by a simple treatment, there has heretofore been known, for example, a process in which a form such as a dot pattern is screen-printed with a printing ink incorporating an inorganic filler, a white pigment and the like on the surface of a light guide formed from a transparent acrylic plate to provide a reflection pattern. A printing ink composed mainly of a liquid with white lacquer diluted with a large amount of lacquer thinner is used as the ink.
When the reflection pattern is formed with the printing ink for the purpose of using it as a light guide, however, it is difficult to sufficiently exhibit the performance, since a molding formed from the alicyclic structure-containing polymer is generally poor in adhesion to the ink layer. In addition, with the production of a large- and thin-sized LCD and the formation of high-quality picture images in recent years, the back light is required to have higher optical properties such as high-intensity luminance and low luminance mottling than before. Besides, the use environment of the back light becomes severer because LCD comes to be used as displays for notebook type computers and car navigations.
Therefore, the printed ink layer of the light guide is forward to form a minute pattern and a complexly shaped pattern and required to cause neither lowering of luminance nor increase of luminance mottling due to interfacial separation of the patterned ink layer and to undergo none of color fading, wrinkling, blistering and the like even when it is left to stand over a long period of time of at least 500 hours under high-temperature and high-humidity environment (for example, under environment of a temperature of 60° C. and a humidity of 90%).
It has been an extremely difficult problem to apply a molding having a patterned ink layer and made of an alicyclic structure-containing polymer to fields of which such high durability is required.
Japanese Patent Application Laid-Open No. 8-5841 has proposed an edge type back light device for liquid crystal making use of a transparent amorphous polyolefin resin plate having a heat distortion temperature of at least 120° C. as a light guide. In this publication, it is described that a dot pattern is formed with a printing ink composed mainly of such a diluted liquid as described above. However, the publication does not describe the details of the printing ink. In addition, Examples of the publication show the results of luminance measurement of back light devices conducted under the same conditions as in real devices. However, this measurement is nothing but a property test under environment as relatively mild as a temperature of 23.7° C. and a humidity of 66% and does not teach the fact that neither lowering of luminance nor increase of luminance mottling is caused even when the back light devices is left to stand over a long period of time under high-temperature and high-humidity environment.
If a pattern such as characters, graduation marks, figures or combinations thereof can be formed with a printing ink not only on the surface of the optical materials such as light guides, but also on the surface of containers such as bottles and bags formed from an alicyclic structure-containing polymer, thereby obtaining an ink layer having excellent adhesion and durability, the development of new uses can be devised.
Japanese Patent Application Laid-Open No. 5-25424 discloses that a coating containing a copolymer composed of a repeating unit derived from a polycyclic (meth)acrylate, a repeating unit derived from an ethylenically unsaturated monomer having a reactive functional group and a repeating unit derived from an ethylenically unsaturated monomer having no reactive functional group is used, whereby a coating film excellent in adhesion can be formed on a molding containing a cycloolefin random copolymer. The Example of this publication shows that the adhesion of the coating film was good when a moisture resistance test that a sample is left to stand for 240 hours under environment of a temperature of 50° C. and a relative humidity of 90% was conducted. However, the coating described in the publication is evenly applied to the surface of the molding to protect the molding and is not a printing ink having good printability.
Japanese Patent Application Laid-Open No. 4-122639 describes that a resin coating is applied to a molding formed from a hydrogenated norbornene resin, thereby improving surface appearance, weather resistance and resistance to damage. However, the resin coating described in the publication is evenly applied to the surface of the molding and is not a printing ink having good printability.